Semiconductor device

ABSTRACT

An object of the present invention is to provide a semiconductor device in which stored data can be held even when power is not supplied for a certain time. Another object is to increase the degree of integration of a semiconductor device and to increase the storage capacity per unit area. A semiconductor device is formed with a material capable of sufficiently reducing off-state current of a transistor, such as an oxide semiconductor material that is a wide-bandgap semiconductor. With the use of a semiconductor material capable of sufficiently reducing off-state current of a transistor, the semiconductor device can hold data for a long time. Furthermore, a wiring layer provided under a transistor, a high-resistance region in an oxide semiconductor film, and a source electrode are used to form a capacitor, thereby reducing the area occupied by the transistor and the capacitor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed invention relates to a semiconductor device including anoxide semiconductor.

2. Description of the Related Art

Memory devices including semiconductor elements are broadly classifiedinto two categories: a volatile memory device that loses stored datawhen power supply is stopped, and a non-volatile memory device thatholds stored data even when power is not supplied.

A typical example of a volatile memory device is a DRAM (dynamic randomaccess memory). A DRAM stores data in such a manner that a transistorincluded in a memory element is selected and charge is stored in acapacitor (e.g., see Patent Document 1).

When data is read from a DRAM, charge in a capacitor is lost on theabove-described principle; thus, another writing operation is necessarywhenever data is read out. Moreover, charge flows into or out of acapacitor even when a transistor included in a memory element is notselected, due to leakage current between a source and a drain in an offstate (off-state current) or the like; therefore, the data storing timeis short. For that reason, another writing operation (refresh operation)is necessary at predetermined intervals, and it is difficult to reducepower consumption sufficiently. Furthermore, since stored data is lostwhen power supply stops, an additional memory device using a magneticmaterial or an optical material is needed in order to store the data fora long time.

Another example of a volatile memory device is an SRAM (static randomaccess memory). An SRAM holds stored data by using a circuit such as aflip-flop and thus does not need refresh operation. This means that anSRAM has an advantage over a DRAM. However, cost per storage capacity isincreased because a circuit such as a flip-flop is used. Moreover, as ina DRAM, stored data in an SRAM is lost when power supply stops.

A structure in which an oxide semiconductor containing indium (In),gallium (Ga), and zinc (Zn) is provided for an active layer of atransistor included in a DRAM has been disclosed (e.g., see PatentDocument 2).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. S53-53277

[Patent Document 2] United States Patent Application Publication No.2011/0156027

SUMMARY OF THE INVENTION

In view of the foregoing problems, an object of one embodiment of thedisclosed invention is to provide a semiconductor device in which storeddata can be held even when power is not supplied for a certain time.

Another object of one embodiment of the disclosed invention is toincrease the degree of integration of a semiconductor device and toincrease storage capacity per unit area.

In the disclosed invention, a semiconductor device is formed with amaterial capable of sufficiently reducing off-state current of atransistor, such as an oxide semiconductor material that is awide-bandgap semiconductor. With the use of a semiconductor materialcapable of sufficiently reducing off-state current of a transistor, datacan be held for a long time. Furthermore, in the disclosed invention, awiring layer provided under a transistor, a high-resistance region in anoxide semiconductor film, and a source electrode are used to form acapacitor, thereby reducing the area occupied by the transistor and thecapacitor.

One embodiment of the disclosed invention is a semiconductor deviceincluding a transistor and a capacitor. The transistor includes an oxidesemiconductor film; a source electrode layer and a drain electrode layerprovided over and in contact with the oxide semiconductor film; a gateelectrode layer provided over and overlapping with the oxidesemiconductor film; and a gate insulating film provided between theoxide semiconductor film and the gate electrode layer. The oxidesemiconductor film includes a channel formation region formed in aregion overlapping with the gate electrode layer; a source region and adrain region formed in a superficial portion of the oxide semiconductorfilm so that the channel formation region is sandwiched between thesource region and the drain region; a high-resistance region overlappingwith and in contact with the source region and the drain region. Thesource region and the drain region have a lower resistance than thechannel formation region and include a metal element. Thehigh-resistance region has a higher resistance than the source regionand the drain region. The source electrode layer is in contact with thesource region of the oxide semiconductor film. The drain electrode layeris in contact with the drain region of the oxide semiconductor film. Thecapacitor comprises a wiring layer provided under the oxidesemiconductor film and overlapping with the high-resistance region; thehigh-resistance region; and the source electrode layer.

In the above semiconductor device, a low-resistance region including adopant may be provided between the channel formation region and thesource region and between the channel formation region and the drainregion. Furthermore, phosphorus or boron is preferably used as thedopant.

In the above semiconductor device, a sidewall insulating film ispreferably provided to be in contact with a side surface of the gateelectrode layer.

In the above semiconductor device, the oxide semiconductor film may beformed over and to be in contact with an insulating film, and the wiringlayer may be embedded in the insulating film.

In the above semiconductor device, the oxide semiconductor filmpreferably includes indium, zinc, and at least one selected from rareearth elements. Alternatively, in the above semiconductor device, theoxide semiconductor film preferably includes indium, zinc, and at leastone selected from zirconium, gadolinium, cerium, and titanium.

In the above semiconductor device, aluminum or magnesium is preferablyused as the metal element.

In this specification and the like, the term “channel length direction”means a direction from a source region (or a source electrode) toward adrain region (or a drain electrode) or the opposite direction, along theshortest path between the source region and the drain region. Further,in this specification and the like, the term “channel width direction”means a direction substantially perpendicular to the channel lengthdirection.

In this specification and the like, the term such as “over” or “below”does not necessarily mean that a component is placed “directly on” or“directly under” another component. For example, the expression “a gateelectrode over a gate insulating layer” can mean the case where there isan additional component between the gate insulating layer and the gateelectrode.

In this specification and the like, the term such as “electrode” or“wiring” does not limit a function of a component. For example, an“electrode” is sometimes used as part of a “wiring”, and vice versa.Furthermore, the term “electrode” or “wiring” can include the case wherea plurality of “electrodes” or “wirings” is formed in an integratedmanner.

Functions of a “source” and a “drain” might interchange when atransistor of opposite polarity is used or the direction of current flowis changed in circuit operation, for example. Therefore, the terms“source” and “drain” can be replaced with each other in thisspecification and the like.

In this specification and the like, the expression “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”include an electrode, a wiring, a switching element such as atransistor, a resistor, an inductor, a capacitor, and an element with avariety of functions.

In one embodiment of the disclosed invention, a semiconductor device ismanufactured with a material capable of sufficiently reducing theoff-state current of a transistor, such as an oxide semiconductormaterial that is a wide-bandgap semiconductor. Owing to thesemiconductor material capable of sufficiently reducing the off-statecurrent of a transistor, stored data can be held for an extremely longtime. In other words, power consumption can be sufficiently reducedbecause refresh operation becomes unnecessary or the frequency ofrefresh operation can be extremely low. Moreover, stored data can beheld for a long time even when power is not supplied for a certain time(note that the potential is preferably fixed).

In the disclosed invention, a wiring layer provided under a transistor,a high-resistance region in an oxide semiconductor film, and a sourceelectrode are used to form a capacitor. Therefore, the area occupied bya transistor and a capacitor can be reduced, whereby the degree ofintegration of a semiconductor device can be enhanced and the storagecapacity per unit area can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a cross-sectional view, a plan view, and a circuitdiagram of a semiconductor device according to one embodiment of thepresent invention;

FIGS. 2A to 2C are a cross-sectional view, a plan view, and a circuitdiagram of a semiconductor device according to one embodiment of thepresent invention;

FIGS. 3A to 3D are cross-sectional views illustrating manufacturingsteps of a semiconductor device according to one embodiment of thepresent invention;

FIGS. 4A to 4C are cross-sectional views illustrating manufacturingsteps of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 5A and 5B are cross-sectional views illustrating manufacturingsteps of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 6A and 6B are cross-sectional views illustrating manufacturingsteps of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 7A to 7C are a cross-sectional view, a plan view, and a circuitdiagram of a semiconductor device according to one embodiment of thepresent invention;

FIG. 8 is a circuit diagram illustrating a semiconductor deviceaccording to one embodiment of the present invention;

FIG. 9 is a block diagram illustrating a semiconductor device accordingto one embodiment of the present invention;

FIG. 10 is a block diagram illustrating a semiconductor device accordingto one embodiment of the present invention; and

FIG. 11 is a block diagram illustrating a semiconductor device accordingto one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description and it will be readily appreciatedby those skilled in the art that modes and details can be modified invarious ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments.

Note that the position, size, range, or the like of each structureillustrated in the drawings and the like is not accurately representedin some cases for easy understanding. Therefore, the disclosed inventionis not necessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, the structure of a semiconductor device according toone embodiment of the present invention is described with reference toFIGS. 1A to 1C and FIGS. 2A to 2C.

<Cross-sectional Structure and Plan View of Semiconductor Device>

FIGS. 1A to 1C illustrate an example of a structure of a semiconductordevice. FIG. 1A is a cross-sectional view of the semiconductor device.FIG. 1B is a plan view of the semiconductor device. In FIG. 1A, A1-A2 isa cross section perpendicular to a channel length direction of atransistor, and B1-B2 is a cross section parallel to the channel lengthdirection of the transistor. The semiconductor device illustrated inFIGS. 1A and 1B includes a transistor 462 and a capacitor 464, whichinclude an oxide semiconductor.

Note that either an n-channel transistor or a p-channel transistor canbe used as the transistor 462. Here, the case where the transistor 462is an n-channel transistor is described.

The transistor 462 includes an oxide semiconductor film 403 providedover a substrate 400 with an insulating film 420 provided therebetween,a source electrode layer (or a drain electrode layer) 442 a and a drainelectrode layer (or a source electrode layer) 442 b provided in contactwith the oxide semiconductor film 403, a gate electrode layer 401overlapping with the oxide semiconductor film 403, and a gate insulatingfilm 402 provided between the oxide semiconductor film 403 and the gateelectrode layer 401. Here, the oxide semiconductor film 403 includes achannel formation region 409, a source region 404 a, a drain region 404b, a high-resistance region 405 a, and a high-resistance region 405 b.The channel formation region 409 is formed in a region overlapping withthe gate electrode layer 401. The source region 404 a and the drainregion 404 b are formed in a superficial portion of the oxidesemiconductor film 403 so that the channel formation region 409 issandwiched between the source region 404 a and the drain region 404 b.The source region 404 a and the drain region 404 b have a higherresistance than the channel formation region 409 and contain a metalelement. The high-resistance region 405 a and the high-resistance region405 b are provided to overlap with the source region 404 a and the drainregion 404 b and have a higher resistance than the source region 404 aand the drain region 404 b. The source electrode layer 442 a is incontact with a source region 404 a of the oxide semiconductor film 403,and the drain electrode layer 442 b is in contact with a drain region404 b of the oxide semiconductor film 403. Furthermore, a sidewallinsulating film 431 may be provided to be in contact with a side surfaceof the gate electrode layer 401.

An oxide semiconductor used for the oxide semiconductor film 403preferably contains at least indium (In) or zinc (Zn). In particular, Inand Zn are preferably contained. In addition to these, one or moreselected from rare earth elements (scandium (Sc), yttrium (Y), andlanthanoid) are preferably contained as a stabilizer for reducingvariation in electric characteristics of a transistor including theoxide semiconductor. It is more preferable that one or more selectedfrom cerium (Ce), neodymium (Nd), and gadolinium (Gd)), which arelanthanoid elements, be contained as the stabilizer. Alternatively,instead of the rare earth elements, one or more selected from zirconium(Zr) and titanium (Ti) may be contained as the stabilizer. With the useof the above-mentioned materials for the oxide semiconductor film 403,in a transistor including the oxide semiconductor film, the amount ofvariation in initial electric characteristics is very small, the ratiobetween on-state current and off-state current increases, the off-statecurrent decreases, and hysteresis is also suppressed.

Since the oxide semiconductor film 403 formed with such materials can bea wide-bandgap semiconductor material which has a large energy gap(e.g., 2.8 eV or higher), the off-state current of the transistor 462including the oxide semiconductor film 403 can be sufficiently small.

Here, it is preferable that the oxide semiconductor film 403 used forthe transistor 462 be highly purified by sufficiently removing animpurity such as hydrogen therefrom and then sufficiently supplyingoxygen thereto. Specifically, the hydrogen concentration in the oxidesemiconductor film 403 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less. Note thatthe hydrogen concentration in the oxide semiconductor film 403 ismeasured by secondary ion mass spectrometry (SIMS). In the oxidesemiconductor film 403 which is highly purified by sufficiently reducingthe hydrogen concentration and in which defect levels in an energy gapdue to oxygen deficiency are reduced by supplying a sufficient amount ofoxygen, the carrier concentration is less than 1×10¹²/cm³, preferablyless than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. Forexample, the off-state current (per unit channel width (1 μm), here) atroom temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) orless, preferably 10 zA or less, more preferably 1 zA or less, still morepreferably 100 yA or less. In this manner, with the use of an i-type(intrinsic) or substantially i-type oxide semiconductor film 403, thetransistor 462 having extremely favorable off-state currentcharacteristics can be obtained.

Note that the oxide semiconductor film 403 is a non-single-crystal oxidesemiconductor film, and may be amorphous or may have crystallinity. Forexample, as the oxide semiconductor film 403 having crystallinity, aCAAC-OS (c-axis aligned crystalline oxide semiconductor) film includingcrystals c-axes of which are substantially perpendicular to the surfacecan be used.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts are includedin an amorphous phase. Note that in most cases, the crystal part fitsinside a cube whose one side is less than 100 nm. From an observationimage obtained with a transmission electron microscope (TEM), a boundarybetween an amorphous part and a crystal part in the CAAC-OS film is notclear. Further, with the TEM, a grain boundary in the CAAC-OS film isnot found. Thus, in the CAAC-OS film, a reduction in electron mobility,due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor layer is higher than that in thevicinity of the surface where the oxide semiconductor layer is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

Note that nitrogen may be substituted for part of oxygen included in theoxide semiconductor film.

In an oxide semiconductor having a crystal part such as the CAAC-OS,defects in the bulk can be further reduced and when the surface flatnessof the oxide semiconductor is improved, mobility higher than that of anoxide semiconductor in an amorphous state can be obtained. In order toimprove the surface flatness, the oxide semiconductor is preferablyformed over a flat surface. Specifically, the oxide semiconductor may beformed over a surface with the average surface roughness (R_(a)) of lessthan or equal to 1 nm, preferably less than or equal to 0.3 nm, morepreferably less than or equal to 0.1 nm.

When a CAAC-OS film is used as the oxide semiconductor film 403, animprovement in carrier-transport properties (e.g., mobility) is expectedand structure stabilization is achieved, which can result in animprovement in characteristics and reliability of an element includingthe oxide semiconductor film 403.

With use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Accordingly, the transistor hashigh reliability.

In the oxide semiconductor film 403 described above, a metal element isadded to superficial portions in the regions between which the channelformation region 409 overlapping with the gate electrode layer 401 issandwiched, so that the source region 404 a and the drain region 404 bare formed. Since the metal element is added to the source region 404 aand the drain region 404 b, the source region 404 a and the drain region404 b have lower resistance than the channel formation region 409. Inthe oxide semiconductor film 403, regions to which the metal element isnot added and which overlap with the source region 404 a and the drainregion 404 b are referred to as high-resistance region 405 a andhigh-resistance region 405 b. The high-resistance region 405 a andhigh-resistance region 405 b have higher resistance than the sourceregion 404 a and the drain region 404 b. Note that in thisspecification, the term “superficial portion” means a portion of theoxide semiconductor film from the surface to a depth corresponding toapproximately 1% to 50% of the thickness of the oxide semiconductorfilm.

As the metal element added to the source region 404 a and the drainregion 404 b, one or more selected from aluminum (Al), titanium (Ti),molybdenum (Mo), tungsten (W), hafnium (Hf), tantalum (Ta), lanthanum(La), barium (Ba), magnesium (Mg), zirconium (Zr), and nickel (Ni) canbe used.

By provision of the source region 404 a and the drain region 404 b whichcontain a metal element and have lower resistance than the channelformation region 409 in the oxide semiconductor film 403 of thetransistor 462, the transistor 462 can have excellent on-statecharacteristics (e.g., on-state current and field effect mobility),leading to high-speed operation and quick response. Furthermore, byelectrically connecting the oxide semiconductor film 403 to the sourceelectrode layer 442 a and the drain electrode layer 442 b in the sourceregion 404 a and the drain region 404 b, contact resistance between theoxide semiconductor film 403 and the source electrode layer 442 a andbetween the oxide semiconductor film 403 and the drain electrode layer442 b can be lowered.

The source electrode layer 442 a, the high-resistance region 405 a, anda wiring layer 422 under the oxide semiconductor film 403 andoverlapping with the high-resistance region 405 a overlap with oneanother, whereby the capacitor 464 is formed. Here, the oxidesemiconductor film 403 is a wide-bandgap semiconductor material asdescribed above, and the high-resistance region 405 a can serve as adielectric substance of the capacitor 464 because of its sufficientlyhigh resistance. In other words, the capacitor 464 is a capacitor inwhich the source electrode layer 442 a serves as one electrode, thehigh-resistance region 405 a serves as a dielectric substance of thecapacitor, and the wiring layer 422 serves as the other electrode. Withsuch a structure, the capacitor 464 can have sufficient capacitance. Inaddition, since the high-resistance region 405 a can be used as adielectric substance of the capacitor, there is no necessity to providean additional insulating film as the dielectric substance; accordingly,the process for manufacturing the semiconductor device can besimplified. Therefore, an improvement in yield of the manufacturingprocess and a reduction in manufacturing cost can be expected.

By the formation of the capacitor 464 in which the source electrodelayer 442 a of the transistor 462 and the oxide semiconductor film 403of the transistor 462 overlap with each other, the area occupied by thesemiconductor device can be reduced. When a memory device in which thesemiconductor devices in this embodiment are arrayed as memory cells ismanufactured, the effect of reduction in area can be obtained inaccordance with the number of memory cells. Thus, the memory device canbe highly integrated effectively, and the storage capacity per unit areacan be increased.

Even when a wide-bandgap oxide semiconductor is used as the oxidesemiconductor film 403 and the high-resistance region 405 a has asufficiently high resistance, a metal element is added to form thesource region 404 a and the drain region 404 b, whereby the transistor462 can have excellent on-state characteristics.

Note that in the semiconductor device in FIG. 1A, the wiring layer 422is formed to be embedded in the insulating film 420 in contact with theoxide semiconductor film 403, be exposed at the top surface, and be incontact with the high-resistance region 405 a; however, the structure isnot limited thereto. For example, a structure in which the wiring layer422 is not exposed at the top surface of the insulating film 420, andthe high-resistance region 405 a and the insulating film 420 whichoverlap with each other are used as the dielectric substance of thecapacitor 464 may be employed.

An insulating film 425 and an insulating film 426 are provided over thetransistor 462 and the capacitor 464. A wiring layer 456 is formed overthe insulating film 425 and the insulating film 426 to be electricallyconnected to the drain electrode layer 442 b through openings formed inthe insulating films 425 and 426 and the like. Here, the wiring layer456 is preferably provided to overlap with at least part of the oxidesemiconductor film 403 of the transistor 462. In addition, an insulatinglayer may be provided over the wiring layer 456.

<Circuit Configuration of Semiconductor Device>

Next, circuit configuration and operation of the semiconductor deviceillustrated in FIGS. 1A and 1B are described with reference to FIG. 1C.

In the semiconductor device illustrated in FIG. 1C, a bit line BL iselectrically connected to a drain electrode of the transistor 462, aword line WL is electrically connected to a gate electrode of thetransistor 462, and a source electrode of the transistor 462 iselectrically connected to a first terminal of the capacitor 464. Apredetermined potential (e.g., ground potential) is applied to a secondterminal of the capacitor 464.

Here, the bit line BL corresponds to the wiring layer 456 in FIG. 1A,the word line WL corresponds to the gate electrode layer 401 in FIG. 1Aor a wiring electrically connected to the gate electrode layer 401, thefirst terminal of the capacitor 464 corresponds to the source electrodelayer 442 a, and the second terminal of the capacitor 464 corresponds tothe wiring layer 422.

Here, the transistor 462 is a transistor including the oxidesemiconductor film 403 which is a wide-bandgap semiconductor material asdescribed above. The transistor including the oxide semiconductor film403 has a feature of significantly small off-state current. For thatreason, a potential of the first terminal of the capacitor 464 (or acharge accumulated in the capacitor 464) can be held for an extremelylong time by turning off the transistor 462. Thus, the semiconductordevice illustrated in FIGS. 1A to 1C serves as a memory element and isreferred to as memory cell below. The memory cells are arrayed to form amemory cell array, whereby a memory device can be formed.

Next, writing and holding of data in the semiconductor device (memorycell) illustrated in FIG. 1C are described.

First, the potential of the word line WL is set to a potential at whichthe transistor 462 is turned on, so that the transistor 462 is turnedon. Thus, the potential of the bit line BL is supplied to the firstterminal of the capacitor 464 (writing). After that, the potential ofthe word line WL is set to a potential at which the transistor 462 isturned off, so that the transistor 462 is turned off. Thus, thepotential of the first terminal of the capacitor 464 is held (holding).

Since the off-state current of the transistor 462 is extremely small,the potential of the first terminal of the capacitor 464 (or the chargeaccumulated in the capacitor) can be held for a long time.

Next, reading of data is described. When the transistor 462 is turnedon, the bit line BL which is in a floating state and the capacitor 464are electrically connected to each other, and the charge isredistributed between the bit line BL and the capacitor 464. As aresult, the potential of the bit line BL is changed. The amount ofchange in potential of the bit line BL varies depending on the potentialof the first terminal of the capacitor 464 (or the charge accumulated inthe capacitor 464).

For example, the potential of the bit line BL after chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 464, C is the capacitance of thecapacitor 464, C_(B) is the capacitance of the bit line BL (hereinafteralso referred to as bit line capacitance), and V_(B0) is the potentialof the bit line BL before the charge redistribution. Therefore, it canbe found that assuming that the memory cell is in either of two statesin which the potentials of the first terminal of the capacitor 464 areV₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case ofholding the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher thanthe potential of the bit line BL in the case of holding the potentialV₀(=(C_(B)×V_(B0)+C×V₀)/(G_(B)+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the semiconductor device illustrated in FIG. 1C canhold charge that is accumulated in the capacitor 464 for a long timebecause the off-state current of the transistor 462 is extremely small.In other words, power consumption can be sufficiently reduced becauserefresh operation becomes unnecessary or the frequency of refreshoperation can be extremely low. Moreover, stored data can be held for along time even when power is not supplied for a certain time.

Note that in the above description, an n-type transistor (an n-channeltransistor) using electrons as carriers is used; however, a p-channeltransistor using holes as carriers can be used instead of an n-channeltransistor.

FIGS. 2A to 2C illustrate a semiconductor device which differs from thesemiconductor device in FIGS. 1A to 1C. FIG. 2A is a cross-sectionalview of the semiconductor device. FIG. 2B is a plan view of thesemiconductor device. FIG. 2C illustrates a circuit configuration of thesemiconductor device. In FIG. 2A, A1-A2 is a cross section perpendicularto a channel length direction of a transistor, and B1-B2 is a crosssection parallel to the channel length direction of the transistor. Thesemiconductor device illustrated in FIGS. 2A to 2C includes a transistor472 and the capacitor 464 each including an oxide semiconductor.

The semiconductor device in FIGS. 2A to 2C includes the transistor 472which differs from the transistor 462 included in the semiconductordevice in FIGS. 1A to 1C. The transistor 472 differs from the transistor462 in that a low-resistance region 406 a containing a dopant is formedbetween the source region 404 a and the channel formation region 409,and a low-resistance region 406 b containing a dopant is formed betweenthe drain region 404 b and the channel formation region 409. Here, thelow-resistance region 406 a and the low-resistance region 406 b eachhave a lower resistance than the channel formation region 409. Note thatthe structure of the transistor 472 is the same as that of thetransistor 462 except the above point, and the structure of thesemiconductor device in FIGS. 2A to 2C other than the transistor 472 isthe same as the structure of the semiconductor device in FIGS. 1A to 1C.Accordingly, the description of FIGS. 1A to 1C can be referred to fordetails.

Here, the dopant is an impurity by which the electrical conductivity ofthe oxide semiconductor film 403 is changed. One or more selected fromthe following can be used as a dopant 421: Group 15 elements (typicalexamples thereof are phosphorus (P), arsenic (As), and antimony (Sb)),boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (C1), titanium (Ti), and zinc(Zn).

By provision of the low-resistance region 406 a and the low-resistanceregion 406 b in this manner, the on-state characteristics (e.g.,on-state current and field effect mobility) of the transistor 472 can befurther improved.

As described above, the off-state current of a transistor including anoxide semiconductor that is a wide-bandgap semiconductor is sufficientlysmall; therefore, the semiconductor device including the transistor inthis embodiment can hold data for an extremely long time. In otherwords, power consumption can be sufficiently reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be held for a long time evenwhen power is not supplied for a certain time (note that the potentialis preferably fixed).

Furthermore, in the semiconductor device in this embodiment, a wiringlayer provided under a transistor, a high-resistance region in an oxidesemiconductor film, and a source electrode are used to form a capacitor.Therefore, the area occupied by a transistor and a capacitor can bereduced, whereby the degree of integration of a semiconductor device canbe enhanced and the storage capacity per unit area can be increased.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

Embodiment 2

In this embodiment, a method for manufacturing the semiconductor devicein Embodiment 1 is described with reference to FIGS. 3A to 3D, FIG. 4Ato 4C, FIGS. 5A and 5B, and FIGS. 6A and 6B.

Description is given below of a process for manufacturing thesemiconductor device including the transistor 462 and the capacitor 464in FIGS. 1A to 1C, with reference to FIGS. 3A to 3D, FIGS. 4A to 4C, andFIGS. 5A and 5B.

First, a layer including a conductive material is formed over thesubstrate 400 having an insulating surface, and the layer including aconductive material is selectively etched to form the wiring layer 422(see FIG. 3A).

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single-crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

The layer including a conductive material can be formed with the use ofa metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as its main component. The layerincluding a conductive material may have a single-layer structure or astacked-layer structure. There is no particular limitation on the methodfor forming the layer including a conductive material, and a variety offilm formation methods such as an evaporation method, a CVD method, asputtering method, or a spin coating method can be employed. Note thatthis embodiment shows an example of the case where the layer including aconductive material is formed with the use of a metal material.

Next, the insulating film 420 is formed to cover the wiring layer 422,and the insulating film 420 is subjected to chemical mechanicalpolishing (CMP) treatment or etching treatment so that the top surfaceof the wiring layer 422 is exposed (see FIG. 3B).

The insulating film 420 can be formed by a plasma CVD method, asputtering method, or the like with the use of silicon oxide, siliconoxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, galliumoxide, silicon nitride, silicon nitride oxide, aluminum nitride,aluminum nitride oxide, or a mixed material of any of these.Alternatively, the insulating film 420 can be formed with the use ofzirconium oxide, cerium oxide, neodymium oxide, gadolinium oxide, or amixed material of any of these. As described above, one or more oxidesselected from the constituent elements of the oxide semiconductor film403 is used for the insulating film 420, whereby an interface betweenthe insulating film 420 and the oxide semiconductor film 403 can be keptwell.

The insulating film 420 may have either a single-layer structure or astacked-layer structure; an oxide insulating film is preferably used asthe film in contact with the oxide semiconductor film 403. In thisembodiment, a silicon oxide film is formed by a sputtering method as theinsulating film 420.

It is preferable that the proportion of oxygen in the insulating film420, which is in contact with the oxide semiconductor film 403, behigher than at least the stoichiometric proportion in the film (bulk).For example, in the case where a silicon oxide film is used as theinsulating film 420, the composition formula is SiO_(2+α) (α>0). Byusing the insulating film 420 described above, oxygen can be supplied tothe oxide semiconductor film 403; accordingly, oxygen vacancy in theoxide semiconductor film 403 can be filled. By supply of oxygen to theoxide semiconductor film 403, the transistor 462 can have favorablecharacteristics.

For example, the insulating film 420 containing a large amount of (anexcess of) oxygen, which is a supply source of oxygen, is provided to bein contact with the oxide semiconductor film 403, whereby oxygen can besupplied from the insulating film 420 to the oxide semiconductor film403. Oxygen may be supplied to the oxide semiconductor film 403 byperforming heat treatment in the state where the oxide semiconductorfilm 403 and the insulating film 420 are at least partly in contact witheach other.

The CMP treatment is treatment for planarizing a surface of an object tobe processed by a combination of chemical and mechanical actions, usingthe surface as a reference. In general, the CMP treatment is treatmentin which a polishing cloth is attached to a polishing stage, thepolishing stage and the object are each rotated or swung while a slurry(an abrasive) is supplied between the object and the polishing cloth,and the surface of the object is polished by chemical reaction of theslurry and the surface of the object and by action of mechanicalpolishing of the object with the polishing cloth.

The CMP treatment may be performed once or plural times. When the CMPtreatment is performed plural times, first polishing is preferablyperformed with a high polishing rate followed by final polishing with alow polishing rate. By performing polishing at different polishingrates, the flatness of the surface of the insulating layer 136 can befurther improved.

Note that in this embodiment, the top surface of the wiring layer 422 isexposed from the insulating film 420, but the structure is not limitedthereto. For example, the top surface of the wiring layer 422 may becovered with the insulating film 420. In that case, the insulating film420 also serves as a dielectric substance of the capacitor 464.

Next, an oxide semiconductor film is formed over the insulating film 420and is patterned into an island shape to overlap with the wiring layer422, so that the oxide semiconductor film 403 is formed.

In order that hydrogen or water does not enter the oxide semiconductorfilm 403 as much as possible during the formation of the oxidesemiconductor film 403, the following is preferably performed as apretreatment of the formation of the oxide semiconductor film 403: thesubstrate provided with the insulating film 420 is heated in apreheating chamber of a sputtering apparatus so that impurities such ashydrogen and moisture adsorbed onto the substrate and the insulatingfilm 420 are removed and exhausted. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable.

An oxide semiconductor used for the oxide semiconductor film 403preferably contains at least indium (In) or zinc (Zn). In particular, Inand Zn are preferably contained. In addition to these, one or moreselected from rare earth elements (scandium (Sc), yttrium (Y), andlanthanoid) are preferably contained as a stabilizer for reducingvariation in electric characteristics of a transistor including theoxide semiconductor. It is more preferable that one or more selectedfrom cerium (Ce), neodymium (Nd), and gadolinium (Gd)), which arelanthanoid elements, be contained as the stabilizer. Alternatively,instead of the rare earth elements, one or more selected from zirconium(Zr) and titanium (Ti) may be contained as the stabilizer. With the useof the above-mentioned materials for the oxide semiconductor film 403,in a transistor including the oxide semiconductor film, the amount ofvariation in initial electric characteristics is very small, the ratiobetween on-state current and off-state current increases, the off-statecurrent decreases, and hysteresis is also suppressed.

Examples of three-component metal oxides, which can be used as the oxidesemiconductor, include In—Zr—Zn-based oxide, In—Ti—Zn-based oxide,In—La—Zn-based oxide, In—Ce—Zn-based oxide, In—Pr—Zn-based oxide,In—Nd—Zn-based oxide, In—Sm—Zn-based oxide, In—Eu—Zn-based oxide,In—Gd—Zn-based oxide, In—Tb—Zn-based oxide, In—Dy—Zn-based oxide,In—Ho—Zn-based oxide, In—Er—Zn-based oxide, In—Tm—Zn-based oxide,In—Yb—Zn-based oxide, In—Lu—Zn-based oxide, In—Sc—Zn-based oxide, andIn—Y—Zn-based oxide.

Note that In—M—Zn-based oxide (note that M represents one or more of theabove-mentioned elements serving as a stabilizer), for example, refersto an oxide containing In, M, and Zn as its main components, and theratio of In to M and Zn is not limited. The In—M—Zn-based oxide maycontain another metal element in addition to In, M, and Zn.

For example, an In—Ce—Zn-based oxide having an atomic ratio ofIn:Ce:Zn=1:1:1, 3:1:2, or 2:1:3, or any of oxides whose composition isin the neighborhood of the compositions can be used. An In—Zr—Zn-basedoxide having an atomic ratio of In:Zr:Zn=1:1:1, 3:1:2, or 2:1:3, or anyof oxides whose composition is in the neighborhood of the compositionscan be used. An In—Ti—Zn-based oxide having an atomic ratio ofIn:Ti:Zn=1:1:1, 3:1:2, or 2:1:3, or any of oxides whose composition isin the neighborhood of the compositions can be used.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained with relativeease.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when the surface flatness of the oxidesemiconductor is improved, mobility higher than that of an oxidesemiconductor in an amorphous state can be obtained. In order to improvethe surface flatness, the oxide semiconductor is preferably formed overa flat surface. Specifically, the oxide semiconductor may be formed overa surface with the average surface roughness (R_(a)) of less than orequal to 1 nm, preferably less than or equal to 0.3 nm, more preferablyless than or equal to 0.1 nm.

Note that, R_(a) is obtained by expanding, into three dimensions,arithmetic mean surface roughness that is defined by JIS B 0601:2001(ISO4287:1997) so as to be applied to a curved surface. The R_(a) can beexpressed as an “average value of the absolute values of deviations froma reference surface to a specific surface” and is defined by the formulabelow.

$\begin{matrix}{R_{a} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f( {x,y} )} - Z_{0}}}{dxdy}}}}}} & \lbrack {{FORMULA}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂,f(x₁, y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). Moreover, S₀represents the area of a rectangle which is obtained by projecting thespecific surface on the xy plane, and Z₀ represents the height of thereference surface (the average height of the specific surface). TheR_(a) can be measured with an atomic force microscope (AFM).

Therefore, planarization treatment may be performed on the region of theinsulating film 420, which is in contact with the oxide semiconductorfilm 403. There is no particular limitation on the planarizationtreatment, and polishing treatment (e.g., CMP treatment), dry etchingtreatment, or plasma treatment can be used. The planarization treatmentcan also serve as the step of exposing the wiring layer 422.

As plasma treatment, reverse sputtering in which an argon gas, forexample, is introduced and plasma is generated can be performed. Thereverse sputtering is a method in which voltage is applied to asubstrate side with use of an RF power supply under an argon atmosphereand plasma is generated in the vicinity of the substrate so that asubstrate surface is modified. Note that instead of an argon atmosphere,a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or thelike may be used. The reverse sputtering can remove particle substances(also referred to as particles or dust) attached to the surface of theinsulating film 420.

As the planarization treatment, polishing treatment, dry etchingtreatment, or plasma treatment may be performed plural times, or thesetreatments may be performed in combination. In the case where thetreatments are combined, the order of steps may be set as appropriate,without particular limitation, depending on the unevenness of thesurface of the insulating film 420.

As the oxide semiconductor film 403, an oxide semiconductor filmincluding a crystal and having crystallinity (crystalline oxidesemiconductor film) can be used. The crystals in the crystalline oxidesemiconductor film may have crystal axes oriented in random directionsor in a certain direction. For example, the above-mentioned CAAC-OSfilm, which is crystalline oxide semiconductor film, can be used as theoxide semiconductor film 403.

The CAAC-OS film can be obtained by any of the following three methods.The first is a method in which an oxide semiconductor film is depositedat a temperature(s) higher than or equal to 200° C. and lower than orequal to 500° C. such that the c-axis is substantially perpendicular tothe surface. The second is a method in which an oxide semiconductor filmis deposited thin, and is subjected to heat treatment at atemperature(s) higher than or equal to 200° C. and lower than or equalto 700° C., so that the c-axis is substantially perpendicular to the topsurface. The third is a method in which a first-layer oxidesemiconductor film is deposited thin, and is subjected to heat treatmentat a temperature(s) higher than or equal to 200° C. and lower than orequal to 700° C., and a second-layer oxide semiconductor film isdeposited thereover, so that the c-axis is substantially perpendicularto the top surface.

The oxide semiconductor film 403 has a thickness greater than or equalto 1 nm and less than or equal to 200 nm (preferably greater than orequal to 5 nm and less than or equal to 30 nm) and can be formed by asputtering method, a molecular beam epitaxy (MBE) method, a CVD method,a pulse laser deposition method, an atomic layer deposition (ALD)method, or the like as appropriate. The oxide semiconductor film 403 maybe formed with a sputtering apparatus which performs deposition in thestate where top surfaces of a plurality of substrates are substantiallyperpendicular to a top surface of a sputtering target.

Note that it is preferable that the oxide semiconductor film be formedunder a condition that much oxygen is contained during film formation(e.g., formed by a sputtering method under a 100% oxygen atmosphere), sothat a film containing much oxygen (preferably including a region whoseoxygen proportion is higher than the stoichiometric proportion in theoxide semiconductor in a crystalline state) is formed.

An oxide semiconductor film is formed by a sputtering method as follows:for example, an In—Ce—Zn film can be formed with the use of an oxidetarget having an atomic ratio of In:Ce:Zn=1:1:1, In:Ce:Zn=3:1:2, orIn:Ce:Zn=2:1:3. Alternatively, an In—Zr—Zn film can be formed with theuse of an oxide target having an atomic ratio of In:Zr:Zn=1:1:1,In:Zr:Zn=3:1:2, or In:Zr:Zn=2:1:3. Further alternatively, an In—Ti—Znfilm can be formed with the use of an oxide target having an atomicratio of In:Ti:Zn=1:1:1, In:Ti:Zn=3:1:2, or In:Ti:Zn=2:1:3.

The filling rate of the metal oxide target is higher than or equal to90% and lower than or equal to 100%, preferably higher than or equal to95% and lower than or equal to 99.9%. With the use of the metal oxidetarget with high filling rate, a dense oxide semiconductor film can beformed.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as asputtering gas for the formation of the oxide semiconductor film.

The substrate is held in a deposition chamber kept under reducedpressure. Then, moisture remaining in the deposition chamber is removed,a sputtering gas from which hydrogen and moisture are removed isintroduced, and the above target is used, so that the oxidesemiconductor film is formed over the substrate 400. In order to removemoisture remaining in the deposition chamber, an entrapment vacuum pumpsuch as a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. As an exhaustion unit, a turbo molecular pump providedwith a cold trap may be used. In the deposition chamber which isexhausted with the cryopump, a hydrogen atom, a compound containing ahydrogen atom, such as water (H₂O), (more preferably, also a compoundcontaining a carbon atom), and the like are removed, whereby theconcentration of an impurity in the oxide semiconductor film formed inthe deposition chamber can be reduced.

The insulating film 420 and the oxide semiconductor film are preferablyformed in succession without exposure to the air. When the insulatingfilm 420 and the oxide semiconductor film are formed in successionwithout exposure to the air, impurities such as hydrogen and moisturecan be prevented from being adsorbed onto a surface of the insulatingfilm 420.

Further, heat treatment may be performed on the oxide semiconductor filmin order to remove excess hydrogen (including water and a hydroxylgroup) (to perform dehydration or dehydrogenation). The temperature ofthe heat treatment is higher than or equal to 300° C. and lower than orequal to 700° C., or lower than the strain point of the substrate. Theheat treatment can be performed under reduced pressure, a nitrogenatmosphere, or the like. For example, after the substrate is put in anelectric furnace which is a kind of heat treatment apparatus, heattreatment is performed on the oxide semiconductor film at 450° C. forone hour under a nitrogen atmosphere.

Note that the heat treatment apparatus used is not limited to anelectric furnace, and a device for heating an object to be processed byheat conduction or heat radiation from a heating element such as aresistance heating element may be alternatively used. For example, arapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can beused. A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas such as argon, is used. An LRTA apparatus is anapparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out of theinert gas.

Such heat treatment for dehydration or dehydrogenation can be performedin the manufacturing process of the transistor 462 anytime after theformation of the oxide semiconductor film 403 and before addition ofoxygen into the oxide semiconductor film 403. For example, the heattreatment may be performed during the formation of a film containing ametal element.

The heat treatment for dehydration or dehydrogenation is preferablyperformed before the oxide semiconductor film is processed into anisland shape, whereby oxygen included in the insulating film 420 can beprevented from being released by the heat treatment.

Note that in the heat treatment, it is preferable that moisture,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. The purity of nitrogen or the rare gas suchas helium, neon, or argon which is introduced into the heat treatmentapparatus is preferably set to 6N (99.9999%) or higher, more preferably7N (99.99999%) or higher (i.e., the impurity concentration is preferably1 ppm or lower, more preferably 0.1 ppm or lower).

After the oxide semiconductor film is heated by the heat treatment, ahigh-purity oxygen gas, a high-purity dinitrogen monoxide gas, or ultradry air (the moisture amount is less than or equal to 20 ppm (−55° C. byconversion into a dew point), preferably less than or equal to 1 ppm, ormore preferably less than or equal to 10 ppb, in the case wheremeasurement is performed with use of a dew point meter of a cavity ringdown laser spectroscopy (CRDS) system) may be introduced into the samefurnace. It is preferable that water, hydrogen, and the like be notcontained in the oxygen gas or the dinitrogen monoxide gas.Alternatively, the purity of the oxygen gas or the dinitrogen monoxidegas which is introduced into the heat treatment apparatus is preferably6N or higher, further preferably 7N or higher (i.e., the impurityconcentration in the oxygen gas or the dinitrogen monoxide gas ispreferably 1 ppm or lower, further preferably 0.1 ppm or lower). By theeffect of the oxygen gas or the dinitrogen monoxide gas, oxygen which isa main component of the oxide semiconductor and which has been reducedat the same time as the step for removing impurities by dehydration ordehydrogenation is supplied, so that the oxide semiconductor film can bea high-purity and electrically i-type (intrinsic) oxide semiconductorfilm.

The oxide semiconductor film 403 is formed by processing the formedoxide semiconductor film into an island shape through a photolithographyprocess. A resist mask for forming the island-shaped oxide semiconductorfilm 403 may be formed by an inkjet method. Formation of the resist maskby an inkjet method needs no photomask, which leads to a reduction inmanufacturing cost.

Note that either dry etching or wet etching, or both may be employed forthe etching of the oxide semiconductor film. As an etchant used for wetetching of the oxide semiconductor film, for example, a mixed solutionof phosphoric acid, acetic acid, and nitric acid, can be used. ITO07N(produced by KANTO CHEMICAL CO., INC.) may also be used.

Next, a gate insulating film 432 is formed over the oxide semiconductorfilm 403.

The surface of the oxide semiconductor film 403 may also be subjected tothe above planarization treatment in order to be more favorably coveredwith the gate insulating film 432. The surface of the oxidesemiconductor film 403 is preferably flat particularly in the case wherea thin insulating film is used as the gate insulating film 432.

The gate insulating film 432 can have a thickness of 1 nm to 100 nm andcan be formed by a sputtering method, an MBE method, a CVD method, apulse laser deposition method, an ALD method, or the like asappropriate. The gate insulating film 432 may be formed with asputtering apparatus which performs film formation with surfaces of aplurality of substrates set substantially perpendicular to a surface ofa sputtering target, which is so called a columnar plasma (CP)sputtering system.

Examples of a material for the gate insulating film 432 include asilicon oxide film, a gallium oxide film, an aluminum oxide film, asilicon nitride film, a silicon oxynitride film, an aluminum oxynitridefilm, and a silicon nitride oxide film. It is preferable that the gateinsulating film 432 include oxygen in a portion which is in contact withthe oxide semiconductor film 403. In particular, it is preferable thatthe proportion of oxygen in the gate insulating film 432 be higher thanat least the stoichiometric proportion in the film (bulk). For example,in the case where a silicon oxide film is used as the gate insulatingfilm 432, the composition formula is SiO_(2+α) (α>0). In thisembodiment, a silicon oxide film of SiO_(2+α) (α>0) is used as the gateinsulating film 432. By using the silicon oxide film as the gateinsulating film 432, oxygen can be supplied to the oxide semiconductorfilm 403, leading to good characteristics. Furthermore, the gateinsulating film 432 is preferably formed in consideration of the size ofa transistor to be formed and the step coverage with the gate insulatingfilm 432.

The gate insulating film 432 can be formed with the use of a high-kmaterial such as hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSiO_(x)N_(y) (x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0,y>0)), or lanthanum oxide, whereby gate leakage current can be reduced.In addition, the gate insulating film 432 can be formed with the use ofzirconium oxide, cerium oxide, neodymium oxide, gadolinium oxide, or amixed material thereof. As described above, one or more oxides selectedfrom the constituent elements of the oxide semiconductor film 403 isused for the gate insulating film 432, whereby an interface between thegate insulating film 432 and the oxide semiconductor film 403 can bekept well. Furthermore, the gate insulating film 432 may have either asingle-layer structure or a stacked-layer structure.

Next, a conductive film is formed by a plasma CVD method, a sputteringmethod, or the like and is selectively patterned to form the gateelectrode layer 401 over the gate insulating film 432 (see FIG. 3C). Thegate electrode layer 401 can be formed with a metal material such asmolybdenum, titanium, tantalum, tungsten, copper, chromium, neodymium,or scandium or an alloy material containing any of these materials asits main component. Alternatively, a semiconductor film typified by apolycrystalline silicon film doped with an impurity element such asphosphorus, or a silicide film such as a nickel silicide film may beused as the gate electrode layer 401. The gate electrode layer 401 mayhave a single-layer structure or a stacked-layer structure. In thisembodiment, the gate electrode layer 401 is formed with tungsten.

The gate electrode layer 401 can also be formed with a conductivematerial such as indium tin oxide, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium zinc oxide, or indium tin oxide to which silicon oxide is added.It is also possible that the gate electrode layer 401 has astacked-layer structure of the above conductive material and the abovemetal material.

As one layer in the stacked-layer structure of the gate electrode layer401, which is in contact with the gate insulating film 432, metal oxidecontaining nitrogen, specifically, an In—Ga—Zn—O film containingnitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O filmcontaining nitrogen, an In—Zn—O film containing nitrogen, a Sn—O filmcontaining nitrogen, an In—O film containing nitrogen, or a metalnitride (e.g., InN or SnN) film can be used. These films have a workfunction of 5 eV or higher, preferably 5.5 eV or higher. In the case ofany of these films is used as the gate electrode layer 401, thethreshold voltage, which is one of the electric characteristics of atransistor, can be positive; accordingly, a so-called normally-offswitching element can be provided.

Next, the gate insulating film 432 is etched using the gate electrodelayer 401 as a mask to expose part of the oxide semiconductor film 403,so that the gate insulating film 402 is formed (see FIG. 3D).

Next, an insulating film is formed over the oxide semiconductor film403, the gate insulating film 402, and the gate electrode layer 401, andis subjected to anisotropic etching, so that a sidewall insulating film431 is formed in a self-aligned manner to be in contact with a sidesurface of the gate electrode layer 401 (see FIG. 4A).

Here, the thickness of the sidewall insulating film 431 is preferably 1nm to 10 nm, more preferably 3 nm to 5 nm. Note that the thickness ofthe sidewall insulating film 431 is not limited thereto and can be setas appropriate.

There is no particular limitation on the sidewall insulating film 431;for example, a silicon oxide film with favorable step coverage, which isformed by reacting TEOS (tetraethyl orthosilicate), silane, or the likewith oxygen, nitrous oxide, or the like can be used. The sidewallinsulating film 431 can be formed by a thermal CVD method, a plasma CVDmethod, an atmospheric pressure CVD method, a bias ECRCVD method, asputtering method, or the like. A silicon oxide film formed by a lowtemperature oxidation (LTO) method may also be used.

Here, the etching of the sidewall insulating film 431 can be performedby, for example, a reactive ion etching (RIE) method.

By providing the sidewall insulating film 431 with a small thickness inthis manner, short-circuit between the gate and the source and the drainof the transistor 462 can be prevented.

Note that the sidewall insulating film 431 is not necessarily providedand the structure in which the transistor 462 does not include thesidewall insulating film 431 may be employed.

Next, over the oxide semiconductor film 403, the gate insulating film402, and the gate electrode layer 401, a metal-element-containing film424 is formed to be in contact with part of the oxide semiconductor film403 while the substrate 400 is being heated (see FIG. 4B). Thetemperature of thermal deposition for the metal-element-containing film424 is higher than or equal to 100° C. and lower than or equal to 700°C., preferably higher than or equal to 200° C. and lower than or equalto 400° C.

Examples of the metal-element-containing film 424 include a metal film,a metal oxide film, and a metal nitride film. It is preferable that ametal element contained in the metal-element-containing film 424 bedifferent from the metal element contained in the channel formationregion 409 in the oxide semiconductor film 403.

As the metal element included in the film containing a metal element,one or more selected from aluminum (Al), titanium (Ti), molybdenum (Mo),tungsten (W), hafnium (Hf), tantalum (Ta), lanthanum (La), barium (Ba),magnesium (Mg), zirconium (Zr), and nickel (Ni) can be used. As the filmcontaining a metal element, a metal film, a metal oxide film, or a metalnitride film containing at least one of the above-described metalelements (e.g., a titanium nitride film, a molybdenum nitride film, or atungsten nitride film) can be used. Further, a dopant such as phosphorus(P) or boron (B) may be included in the film containing a metal element.In this embodiment, the metal-element-containing film 424 has electricalconductivity.

The metal-element-containing film 424 can be formed by a plasma CVDmethod, a sputtering method, an evaporation method, or the like. Thethickness of the metal-element-containing film 424 may be greater thanor equal to 5 nm and less than or equal to 30 nm.

In this embodiment, a 10-nm-thick aluminum film is formed by asputtering method as the metal-element-containing film 424.

The thermal deposition may be performed under an atmosphere of nitrogen,ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). Note that it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen,ultra-dry air, or a rare gas. It is also preferable that the purity ofnitrogen or a rare gas which is introduced into a heat treatmentapparatus be 6N (99.9999%) or higher, preferably 7N (99.99999%) orhigher (i.e., the impurity concentration is 1 ppm or lower, preferably0.1 ppm or lower). Alternatively, the thermal deposition may beperformed under reduced pressure or in a vacuum.

By the thermal deposition of the metal-element-containing film 424, themetal element(s) contained in the metal-element-containing film 424 isintroduced into the oxide semiconductor film 403. Thus, the channelformation region 409 is formed in a self-aligned manner in a region ofthe oxide semiconductor film 403, which overlaps with the gate electrodelayer 401. A metal element is added to superficial portions in theregions between which the channel formation region 409 is sandwiched inthe channel length direction, thereby forming the source region 404 aand the drain region 404 b having a lower resistance than the channelformation region 409. In the oxide semiconductor film 403, regions towhich the metal element is not added and which overlap with the sourceregion 404 a and the drain region 404 b are referred to ashigh-resistance region 405 a and high-resistance region 405 b. Thehigh-resistance region 405 a and high-resistance region 405 b havehigher resistance than the source region 404 a and the drain region 404b.

By provision of the source region 404 a and the drain region 404 b whichcontain a metal element and have lower resistance than the channelformation region 409 in the oxide semiconductor film 403 of thetransistor 462, the transistor 462 can have excellent on-statecharacteristics (e.g., on-state current and field effect mobility),leading to high-speed operation and quick response.

Note that in the step illustrated in FIG. 4B, the metal element isintroduced into the oxide semiconductor film 403 by the thermaldeposition of the metal-element-containing film 424, but one embodimentof the present invention is not limited thereto. For example, themetal-element-containing film 424 may be formed at room temperature, ormay be formed by being heated at temperature at which the metal elementis not introduced into the oxide semiconductor film 403 (e.g.,temperature lower than 100° C.) and be subjected to heat treatment afterthe formation. The heating temperature may be set to higher than orequal to 100° C. and lower than or equal to 700° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C.

Next, the metal-element-containing film 424 is removed by etching (seeFIG. 4C). For the etching of the metal-element-containing film 424, wetetching or dry etching may be used, and etching conditions may be set asappropriate. Here, since wet etching can be employed, themetal-element-containing film 424 can be removed without performingplasma treatment; accordingly, the semiconductor device can be preventedfrom being broken due to ESD caused by plasma damage.

Next, a conductive layer for forming a source electrode layer and adrain electrode layer is formed over the oxide semiconductor film 403and the like and is processed into the source electrode layer 442 a andthe drain electrode layer 442 b (see FIG. 5A). Here, the sourceelectrode layer 442 a is in contact with the source region 404 a in theoxide semiconductor film 403, and the drain electrode layer 442 b is incontact with the drain region 404 b in the oxide semiconductor film 403.

The conductive layer can be formed by a PVD method or a CVD method. As amaterial for the conductive layer, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloycontaining any of these elements as a component; or the like can beused. Further, one or more materials selected from manganese, magnesium,zirconium, beryllium, neodymium, and scandium may be used.

The conductive layer may have a single-layer structure or astacked-layer structure including two or more layers. For example, theconductive layer can have a single-layer structure of a titanium film ora titanium nitride film, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, or a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order.

Alternatively, the conductive layer may be formed with the use ofconductive metal oxide. Examples of the conductive metal oxide includeindium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium tinoxide (In₂O₃—SnO₂, which is abbreviated to ITO in some cases), indiumzinc oxide (In₂O₃—ZnO), and any of these metal oxide materials whichcontains silicon or silicon oxide.

Note that in the steps illustrated in FIG. 4C and FIG. 5A, themetal-element-containing film 424 is completely removed, and then, thesource electrode layer 442 a and the drain electrode layer 442 b areformed, but one embodiment of the present invention is not limitedthereto. For example, the metal-element-containing film 424 may bepartly removed, and the remaining metal-element-containing film 424 maybe used as the source electrode layer 442 a and the drain electrodelayer 442 b.

Then, the insulating film 425 is formed to cover the transistor 462.

The insulating film 425 is preferably formed by a method such as asputtering method, by which impurities such as water and hydrogen doesnot enter the insulating film 425, as appropriate. It is preferable thatthe insulating film 425 include much oxygen because it serves as asupply source of oxygen to the oxide semiconductor film 403.

In this embodiment, a 100-nm-thick silicon oxide film is formed as theinsulating film 425 by a sputtering method. The silicon oxide film canbe formed by a sputtering method under a rare gas (typically, argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere containing arare gas and oxygen.

To remove residual moisture from the deposition chamber of theinsulating film 425 in a manner similar to that of the formation of theoxide semiconductor film, an entrapment vacuum pump (e.g., a cryopump)is preferably used. When the insulating film 425 is formed in thedeposition chamber exhausted with a cryopump, the impurity concentrationin the insulating film 425 can be reduced. As an exhaustion unit forremoving residual moisture in the deposition chamber of the insulatingfilm 425, a turbo molecular pump provided with a cold trap may be used.

A high-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride are removed is preferably used as asputtering gas used in the formation of the insulating film 425.

In the case where the insulating film 425 has a stacked-layer structure,a silicon oxide film and an inorganic insulating film typified by analuminum oxide film, a silicon oxynitride film, an aluminum oxynitridefilm, or a gallium oxide film can be used. For example, as theinsulating film 425, a stacked layer of a silicon oxide film and analuminum oxide film can be used.

In order to reduce surface unevenness caused by a transistor, theinsulating film 426 serving as a planarization insulating film may beformed. For the insulating film 426, an organic material such as apolyimide-based resin, an acrylic-based resin, or abenzocyclobutene-based resin can be used. Other than such organicmaterials, it is possible to use a low-dielectric constant material (alow-k material) or the like. Alternatively, the insulating film 426 maybe formed by stacking plural insulating films formed with any of thesematerials.

After the formation of the insulating film 425, heat treatment may beperformed under an inert gas atmosphere or an oxygen atmosphere. Theheat treatment temperature is preferably higher than or equal to 200° C.and lower than or equal to 450° C., and more preferably higher than orequal to 250° C. and lower than or equal to 350° C. With such a heattreatment, variations in electric characteristics of the transistor 462can be reduced. Further, in the case where the insulating film 420, thegate insulating film 402, or the insulating film 425 contains oxygen,oxygen can be supplied to the oxide semiconductor film 403 to filloxygen vacancy in the oxide semiconductor film 403. As described above,the heat treatment has an effect of supplying oxygen; therefore, theheat treatment can also be referred to as supply of oxygen. The heattreatment for the metal-element-containing film 424 can also function asthe supply of oxygen.

Finally, an opening reaching the drain electrode layer 442 b is formedin the insulating film 425 and the insulating film 426. Over theinsulating film 425 and the insulating film 426, the wiring layer 456 isformed to be in contact with the drain electrode layer 442 b through theopening (see FIG. 5B).

Examples of a conductive film used for the wiring layer 456 include ametal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, and a metal nitride film containing any of the above elements asa component (e.g., a titanium nitride film, a molybdenum nitride film,or a tungsten nitride film). A metal film having a high melting pointsuch as Ti, Mo, or W, or a metal nitride film of any of these elements(a titanium nitride film, a molybdenum nitride film, or a tungstennitride film) may be stacked on one of or both of a lower side and anupper side of the metal film of Al, Cu, or the like.

A resist mask is formed over the conductive film through aphotolithography process and selective etching is performed, whereby thewiring layer 456 can be formed.

In this manner, the transistor 462 and the capacitor 464 can be formed.The transistor 462 includes the oxide semiconductor film 403 over thesubstrate 400 with the insulating film 420 provided therebetween, thesource electrode layer 442 a and the drain electrode layer 442 b formedin contact with the oxide semiconductor film 403, the gate electrodelayer 401 formed over and to overlap with the oxide semiconductor film403, and the gate insulating film 402 provided between the oxidesemiconductor film 403 and the gate electrode layer 401. The capacitor464 includes the source electrode layer 442 a, the high-resistanceregion 405 a, and the wiring layer 422 under the oxide semiconductorfilm 403 and overlapping with the high-resistance region 405 a.

In the oxide semiconductor film 403 which is highly purified and whoseoxygen vacancy is filled, impurities such as hydrogen and water aresufficiently removed; the hydrogen concentration in the oxidesemiconductor film 403 is less than or equal to 5×10¹⁹ atoms/cm³,preferably less than or equal to 5×10¹⁸ atoms/cm³, more preferably lessthan or equal to 5×10¹⁷ atoms/cm³.

The amount of carriers in the oxide semiconductor film 403 is extremelysmall (close to zero). The carrier concentration is less than1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than1.45×10¹⁰/cm³.

The transistor 462 formed according to this embodiment, which includesthe highly purified oxide semiconductor film 403 containing excessiveoxygen with which oxygen vacancy is filled, can have a current value inan off state (off-state current value) of less than or equal to 100 zAper micrometer of channel width at room temperature, preferably lessthan or equal to 10 zA/μm, more preferably less than or equal to 1zA/μm, even more preferably less than or equal to 100 yA/μm.

As described above, the off-state current of a transistor including anoxide semiconductor that is a wide-bandgap semiconductor is sufficientlysmall; therefore, the semiconductor device including the transistor inthis embodiment can hold data for an extremely long time. In otherwords, power consumption can be sufficiently reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be held for a long time evenwhen power is not supplied for a certain time (note that the potentialis preferably fixed).

Furthermore, in the semiconductor device in this embodiment, a wiringlayer provided under a transistor, a high-resistance region in an oxidesemiconductor film, and a source electrode are used to form a capacitor.Therefore, the area occupied by a transistor and a capacitor can bereduced, whereby the degree of integration of a semiconductor device canbe enhanced and the storage capacity per unit area can be increased.

The process for manufacturing the semiconductor device including thetransistor 472 and the capacitor 464 illustrated in FIGS. 2A to 2C isdescribed with reference to FIGS. 6A and 6B. The transistor 472 differsfrom the transistor 462 in that a low-resistance region 406 a containinga dopant is formed between the source region 404 a and the channelformation region 409, and a low-resistance region 406 b containing adopant is formed between the drain region 404 b and the channelformation region 409. Note that the structure of the transistor 472 isthe same as that of the transistor 462 except the above point, and thestructure of the semiconductor device in FIGS. 2A to 2C other than thetransistor 472 is the same as the structure of the semiconductor devicein FIGS. 1A to 1C.

First, the steps from FIG. 3A to FIG. 5A are conducted in the samemanners as those for manufacturing the semiconductor device in FIGS. 1Ato 1C.

Next, the dopant 421 is selectively added to the oxide semiconductorfilm 403 while the gate insulating film 402, the gate electrode layer401, the source electrode layer 442 a, and the drain electrode layer 442b are used as a mask, whereby the low-resistance region 406 a containingthe dopant is formed between the source region 404 a and the channelformation region 409 and the low-resistance region 406 b containing thedopant is formed between the drain region 404 b and the channelformation region 409 (see FIG. 6A).

The dopant 421 is an impurity by which the electrical conductivity ofthe oxide semiconductor film 403 is changed. One or more selected fromthe following can be used as the dopant 421: Group 15 elements (typifiedby phosphorus (P), arsenic (As), and antimony (Sb)), boron (B), aluminum(Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium (In),fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn).

As the method for adding the dopant 421, an ion implantation method, anion doping method, a plasma immersion ion implantation method, or thelike can be used. In that case, it is preferable to use a single ion ofthe dopant 421 or a hydride ion, a fluoride ion, or a chloride ionthereof

The addition of the dopant 421 may be controlled by setting the additionconditions such as the accelerated voltage and the dosage, or thethickness of the metal-element-containing film 424 through which thedopant 421 passes, as appropriate. For example, for addition of an boronion by an ion implantation method using boron, the accelerated voltageand the dosage may be set to 15 kV and 1×10¹⁵ ions/cm², respectively.The dosage may be set to greater than or equal to 1×10¹³ ions/cm² andless than or equal to 5×10¹⁶ ions/cm².

The concentration of the dopant 421 in the source region or the drainregion is preferably higher than or equal to 5×10¹⁸/cm³ and lower thanor equal to 1×10²²/cm³.

The addition of the dopant 421 into the oxide semiconductor film 403 maybe performed plural times, and plural kinds of dopant may be used.

After the addition of the dopant 421, heat treatment may be performedthereon. The heat treatment is preferably performed at a temperaturehigher than or equal to 300° C. and lower than or equal to 700° C. (morepreferably higher than or equal to 300° C. and lower than or equal to450° C.) under a nitrogen atmosphere, reduced pressure, or the air(ultra dry air).

In the case where the oxide semiconductor film 403 is a crystallineoxide semiconductor film, the oxide semiconductor film 403 may be partlyamorphized by the addition of the dopant 421. In that case, thecrystallinity of the oxide semiconductor film 403 can be recovered byheat treatment performed after the addition of the dopant 421.

The insulating film 425, the insulating film 426, and the wiring layer456 can be formed by the same method as that illustrated in FIG. 5B (seeFIG. 6B).

By provision of the low-resistance region 406 a and the low-resistanceregion 406 b in this manner, on-state characteristics (e.g., on-statecurrent and field effect mobility) of the transistor 472 can be furtherimproved.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

Embodiment 3

In this embodiment, an example of a semiconductor device which includesa transistor 482 having a structure similar to the structure of thetransistor 472 described in the above embodiment, which can hold storeddata even when power is not supplied for a certain time, and which hasno limitation on the number of write cycles, is described with referenceto drawings.

Since the off-state current of the transistor 482 is small as thetransistor 472, stored data can be held for a long time owing to such atransistor. In other words, power consumption can be sufficientlyreduced because a semiconductor device in which refresh operation isunnecessary or the frequency of refresh operation is extremely low canbe provided.

FIGS. 7A to 7C illustrate an example of a structure of a semiconductordevice. FIG. 7A is a cross-sectional view, FIG. 7B is a plan view, andFIG. 7C is a circuit diagram of the semiconductor device. Here, FIG. 7Acorresponds to a cross section taken along lines C1-C2 and D1-D2 of FIG.7B.

The semiconductor device illustrated in FIGS. 7A and 7B includes, in thelower portion, a transistor 480 including a semiconductor material(e.g., silicon) other than an oxide semiconductor, and in the upperportion, a transistor 482 including an oxide semiconductor material likethe transistor 472. The transistor 482 has the same structure as thetransistor 472 described in the above embodiment; thus, in descriptionof FIGS. 7A and 7B, the same reference numerals are used for the sameparts as those in FIGS. 2A and 2B.

Here, a material of an active layer of the transistor 480 and a materialof an active layer of the transistor 482 preferably have differentbandgaps. For example, the active layer of the transistor 480 may be asemiconductor (e.g., silicon) and the active layer of the transistor 482may be an oxide semiconductor. With the use of silicon or the like, thetransistor 480 including a material other than an oxide semiconductorcan operate at high speed easily. On the other hand, the transistor 482including an oxide semiconductor enables charge to be held for a longtime owing to its characteristics.

Although the transistors 480 and 482 are n-channel transistors here, itis needless to say that p-channel transistors can be used. The technicalnature of the disclosed invention is to use a wide-bandgap semiconductorin the transistor 482 so that data can be held. Therefore, it is notnecessary to limit a specific structure of the semiconductor device,such as a material of the semiconductor device or a structure of thesemiconductor device, to the structure described here.

The transistor 480 in FIG. 7A includes a channel formation region 116provided in a substrate 100 including a semiconductor material (e.g.,silicon), impurity regions 120 by which the channel formation region 116is sandwiched, metal compound regions 124 in contact with the impurityregions 120, a gate insulating layer 108 provided over the channelformation region 116, and a gate electrode layer 110 provided over thegate insulating layer 108. Here, the gate electrode layer 110 ispreferably formed with the same material as the metal-element-containingfilm 424 described in the above embodiment.

Further, the substrate 100 is provided with an element isolationinsulating layer 106 which surrounds the transistor 480. An insulatingfilm 130 is provided to cover the transistor 480. Note that for highintegration, it is preferable that, as in FIG. 7A, the transistor 480 donot have a sidewall insulating layer. On the other hand, when thecharacteristics of the transistor 480 have priority, the sidewallinsulating layer may be formed on a side surface of the gate electrodelayer 110 and the impurity regions 120 may include a region having adifferent impurity concentration.

As illustrated in FIG. 7A, the transistor 482 includes the oxidesemiconductor film 403 provided over the insulating film 130, the drainelectrode layer 442 b provided in contact with the oxide semiconductorfilm 403, the gate electrode layer 401 provided over and to overlap withthe oxide semiconductor film 403, and the gate insulating film 402provided between the oxide semiconductor film 403 and the gate electrodelayer 401. The insulating film 412 is formed over the oxidesemiconductor film 403 with the same material and in the same step asthe gate insulating film 402. The electrode layer 411 is formed over theinsulating film 412 with the same material in the same step as the gateelectrode layer 401. Here, the oxide semiconductor film 403 includes thechannel formation region 409, the low-resistance region 406 a and thelow-resistance region 406 b, the drain region 404 b, and thehigh-resistance region 405 b. The channel formation region 409 is formedto overlap with the gate electrode layer 401. The low-resistance region406 a and the low-resistance region 406 b are formed so that the channelformation region 409 is sandwiched therebetween. The low-resistanceregion 406 a and the low-resistance region 406 b have a lower resistancethan the channel formation region 409 and contain a dopant. The drainregion 404 b is provided in a superficial portion of the oxidesemiconductor film 403 so that the low-resistance region 406 b issandwiched between the channel formation region 409 and the drain region404 b. The high-resistance region 405 b is provided to overlap with thedrain region 404 b and have a higher resistance than the drain region404 b. Furthermore, the oxide semiconductor film 403 includes ahigh-resistance region 407 and a source region 408. The high-resistanceregion 407 is provided to overlap with the electrode layer 411 so thatthe low-resistance region 406 a is sandwiched between the channelformation region 409 and the high-resistance region 407. The sourceregion 408 is provided to overlap with the high-resistance region 407and has a lower resistance than the high-resistance region 407 of theoxide semiconductor film 403. Here, the gate electrode layer 110 is incontact with the source region 408 of the oxide semiconductor film 403,and the drain electrode layer 442 b is in contact with the drain region404 b of the oxide semiconductor film 403. Furthermore, a sidewallinsulating film may be provided to be in contact with a side surface ofthe gate electrode layer 401, as in the transistors 462 and 472.

Here, the oxide semiconductor film 403, the low-resistance region 406 a,the low-resistance region 406 b, the high-resistance region 405 a, thedrain region 404 b, the gate electrode layer 401, the gate insulatingfilm 402, and the drain electrode layer 442 b can be formed using thematerials and methods described in the above embodiment.

The high-resistance region 407 included in the oxide semiconductor film403 is formed in a self-aligned manner using the electrode layer 411 asa mask, like the channel formation region 409. Since the gate electrodelayer 110 is formed of the same material as the metal-element-containingfilm 424, the source region 408 is formed in a region where the gateelectrode layer 110 is in contact with the oxide semiconductor film 403,as in the case of the source region 404 a described in the aboveembodiment. The source region 408 of the oxide semiconductor film 403 iselectrically connected to the gate electrode layer 110; in other words,the gate electrode layer 110 also serves as a source electrode of thetransistor 482.

The gate electrode layer 110, the high-resistance region 407, theinsulating film 412, and the electrode layer 411 overlap with oneanother, whereby a capacitor 484 is formed. Here, the oxidesemiconductor film 403 is a wide-bandgap semiconductor as described inthe above embodiment, and the high-resistance region 407 can serve as adielectric substance of the capacitor 484 because of its sufficientlyhigh resistance. In other words, the capacitor 484 is a capacitor inwhich the gate electrode layer 110 serves as one electrode, thehigh-resistance region 407 and the insulating film 412 serve as adielectric substance of the capacitor, and the electrode layer 411serves as the other electrode. With such a structure, the capacitor 484can have sufficient capacitance.

By forming the capacitor 484 so as to overlap with the oxidesemiconductor film 403 of the transistor 482, the area occupied by thesemiconductor device can be reduced. When a memory device in which thesemiconductor devices in this embodiment are arrayed as memory cells ismanufactured, the effect of reduction in area can be obtained inaccordance with the number of memory cells. Thus, the memory device canbe highly integrated effectively, and the storage capacity per unit areacan be increased.

Even when a wide-bandgap oxide semiconductor is used as the oxidesemiconductor film 403 and the high-resistance region 407 has asufficiently high resistance, a metal element is added to form thesource region 408 and the drain region 404 b, whereby the transistor 462can have excellent on-state characteristics.

By provision of the low-resistance region 406 a and the low-resistanceregion 406 b, the on-state characteristics (e.g., on-state current andfield effect mobility) of the transistor 482 can be further improved.

The insulating film 425, the insulating film 426, and the wiring layer456 are formed over the transistor 482 and the capacitor 484. Thematerials and methods described in the above embodiments can be referredto for the formation of the insulating films 425 and 426, and the wiringlayer 456.

In FIGS. 7A and 7B, the transistor 480 and the transistor 482 areprovided to overlap with each other at least partly. The source regionor the drain region of the transistor 480 is preferably provided tooverlap with part of the oxide semiconductor film 403. In addition, thetransistor 482 and the capacitor 484 are preferably provided to overlapwith at least part of the transistor 480. With such a planar layout, thearea occupied by the semiconductor device can be reduced; thus, higherintegration can be achieved.

FIG. 7C illustrates an example of a circuit configuration correspondingto FIGS. 7A and 7B.

In FIG. 7C, a first wiring (1st Line) is electrically connected to asource electrode of the transistor 480. A second wiring (2nd Line) iselectrically connected to a drain electrode of the transistor 480. Athird wiring (3rd Line) is electrically connected to the other of thesource electrode and the drain electrode of the transistor 482, and afourth wiring (4th Line) is electrically connected to a gate electrodeof the transistor 482. A gate electrode of the transistor 480 and one ofa source electrode and a drain electrode of the transistor 482 areelectrically connected to one electrode of the capacitor 484. A fifthwiring (5th line) is electrically connected to the other electrode ofthe capacitor 484.

The semiconductor device in FIG. 7C utilizes an advantage that thepotential of the gate electrode of the transistor 480 can be held,whereby writing, holding, and reading of data can be performed asdescribed below.

Writing and holding of data are described. First, the potential of thefourth wiring is set to a potential at which the transistor 482 isturned on, so that the transistor 482 is turned on. Thus, the potentialof the third wiring is supplied to a node (also referred to as node FG)to which the gate electrode of the transistor 480 and one electrode ofthe capacitor 484 are connected. In other words, a predetermined chargeis supplied to the gate electrode of the transistor 480 (i.e., writingof data). Here, charge for supplying either of two different potentiallevels (hereinafter referred to as low-level charge and high-levelcharge) is given. After that, the potential of the fourth wiring is setto a potential at which the transistor 482 is turned off, so that thetransistor 482 is turned off Thus, the charge supplied to the gateelectrode of the transistor 480 is held (i.e., holding of data).

The off-state current of the transistor 482 is extremely small;accordingly, the charge in the gate electrode of the transistor 480 isheld for a long time.

Next, reading of data is described. When an appropriate potential(reading potential) is applied to the fifth wiring in a state where apredetermined potential (constant potential) is applied to the firstline, the potential of the second wiring varies depending on the amountof charge held in the gate electrode of the transistor 480. This isbecause in general, when the transistor 480 is an n-channel transistor,an apparent threshold voltage V_(th) _(_) _(H) in the case where ahigh-level charge is given to the gate electrode of the transistor 480is lower than an apparent threshold voltage V_(th) _(_) _(L) in the casewhere a low-level charge is given to the gate electrode of thetransistor 480. Here, the apparent threshold voltage refers to thepotential of the fifth wiring, which is needed to turn on the transistor480. Thus, the potential of the fifth wiring is set to a potential V₀which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby chargegiven to the gate electrode of the transistor 480 can be determined. Forexample, in the case where a high-level charge is given in writing, whenthe potential of the fifth wiring is set to V₀ (>V_(th) _(_) _(H)), thetransistor 480 is turned on. In the case where low-level charge is givenin writing, even when the potential of the fifth wiring is set to V₀(<V_(th) _(_) _(L)), the transistor 480 remains in an off state.Therefore, the stored data can be read by the potential of the secondwiring.

Note that in the case where memory cells are arrayed, it is necessary toread data only from a predetermined memory cell. In such a case wheredata of the other memory cells is not read, a potential at which thetransistor 480 is turned off, that is, a potential smaller than V_(th)_(_) _(H) may be given to the fifth wiring regardless of the state ofthe gate electrode of the transistor 480. Alternatively, a potential atwhich the transistor 480 is turned on, that is, a potential higher thanV_(th) _(_) _(L) may be given to the fifth wiring regardless of thestate of the gate electrode of the transistor 480.

In the semiconductor device described in this embodiment, the transistorhaving an extremely small off-state current, in which an oxidesemiconductor which is a wide-bandgap semiconductor is used for achannel formation region, is used, whereby stored data can be held foran extremely long time. In other words, power consumption can besufficiently reduced because refresh operation becomes unnecessary orthe frequency of refresh operation can be extremely low. Moreover,stored data can be held for a long time even when power is not suppliedfor a certain time (note that the potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus, a problem such as deteriorationof a gate insulating layer does not occur at all. In other words, thesemiconductor device according to one embodiment of the presentinvention does not have a limit on the number of write cycles, which isa problem in a conventional nonvolatile memory, and reliability thereofis drastically improved. Furthermore, data is written depending on theon state and the off state of the transistor, whereby high-speedoperation can be easily achieved.

Since a transistor including a material other than an oxidesemiconductor can operate at sufficiently high speed, a semiconductordevice can perform operation (e.g., reading data) at sufficiently highspeed in combination of a transistor including an oxide semiconductor.Furthermore, with the use of a transistor including a material otherthan an oxide semiconductor, a variety of circuits (e.g., a logiccircuit and a driver circuit) which is required to operate at high speedcan be favorably obtained.

As described above, a semiconductor device having a novel feature can beachieved by being provided with both a transistor including a materialother than an oxide semiconductor (in a broader sense, a transistorcapable of operating at sufficiently high speed) and a transistorincluding an oxide semiconductor (in a broader sense, a transistor whoseoff-state current is sufficiently small).

Furthermore, in the semiconductor device in this embodiment, the gateelectrode of the transistor in the lower portion, the high-resistanceregion in the oxide semiconductor film, and an electrode formed from thesame layer as the gate electrode layer of the transistor in the upperportion are used to form a capacitor. Therefore, the area occupied by atransistor and a capacitor can be reduced, whereby the degree ofintegration of a semiconductor device can be enhanced and the storagecapacity per unit area can be increased.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

Embodiment 4

In this embodiment, examples of application of the semiconductor devicedescribed in any of the above embodiments to portable devices such ascellular phones, smartphones, or electronic books are described withreference to FIG. 8, FIG. 9, FIG. 10, and FIG. 11.

In a portable device such as a cellular phone, a smartphone, or anelectronic book, a DRAM or the like is used so as to store image datatemporarily. The reason why a DRAM is used is that a flash memory isslow in responding and is not suitable for image processing. On theother hand, a DRAM has the following characteristics when used fortemporary storage of image data.

In a DRAM, as illustrated in FIG. 8, a memory cell includes a transistor811 and a storage capacitor 812, which are driven with an X decoder 813and a Y decoder 814. One cell is configured with one transistor and onecapacitor and has a small area. The area of a memory cell in a DRAM isgenerally 10 F² or less. Note that the DRAM needs to be refreshedperiodically and consumes electric power even when a rewriting operationis not performed.

On the other hand, the memory cell of the semiconductor device describedin any of the above embodiments does not need to be refreshedfrequently. Therefore, the area of a memory cell can be decreased, andpower consumption can be reduced.

Next, FIG. 9 is a block diagram of a portable device. The portabledevice illustrated in FIG. 9 includes an RF circuit 901, an analogbaseband circuit 902, a digital baseband circuit 903, a battery 904, apower supply circuit 905, an application processor 906, a flash memory910, a display controller 911, a memory circuit 912, a display 913, atouch sensor 919, an audio circuit 917, a keyboard 918, and the like.The display 913 includes a display portion 914, a source driver 915, anda gate driver 916. The application processor 906 includes a CPU 907, aDSP 908, and an interface (IF) 909. In general, the memory circuit 912includes an SRAM or a DRAM. By employing the semiconductor devicedescribed in any of the above embodiments for that portion, data can bewritten and read at high speed and can be held for a long time, andpower consumption can be sufficiently reduced.

FIG. 10 illustrates an example of using the semiconductor devicedescribed in any of the above embodiments in a memory circuit 950 for adisplay. The memory circuit 950 illustrated in FIG. 10 includes a memory952, a memory 953, a switch 954, a switch 955, and a memory controller951. The memory circuit 950 is connected to a display controller 956that reads and controls image data input through a signal line (inputimage data) and data stored in the memory 952 and the memory 953 (storedimage data), and is also connected to a display 957 that displays animage based on a signal input from the display controller 956.

First, image data (input image data A) is produced by an applicationprocessor (not illustrated). The input image data A is stored in thememory 952 through the switch 954. Then, the image data stored in thememory 952 (stored image data A) is transmitted to the display 957through the switch 955 and the display controller 956, and is displayedon the display 957.

When the input image data A remains unchanged, the stored image data Ais read from the memory 952 through the switch 955 by the displaycontroller 956 normally at a frequency of approximately 30 Hz to 60 Hz.

Next, for example, when a user performs an operation to rewrite a screen(i.e., when the input image data A is changed), the applicationprocessor produces new image data (input image data B). The input imagedata B is stored in the memory 953 through the switch 954. Also duringthat time, the stored image data A is regularly read from the memory 952through the switch 955. After the completion of storing the new imagedata (the stored image data B) in the memory 953, from the next framefor the display 957, the stored image data B starts to be read,transmitted to the display 957 through the switch 955 and the displaycontroller 956, and displayed on the display 957. This reading operationcontinues until the next new image data is stored in the memory 952.

By alternately writing and reading image data to and from the memory 952and the memory 953 as described above, images are displayed on thedisplay 957. Note that the memory 952 and the memory 953 are not limitedto separate memories, and a single memory may be divided and used. Byemploying the semiconductor device described in any of the aboveembodiments for the memory 952 and the memory 953, data can be writtenand read at high speed and held for a long time, and power consumptioncan be sufficiently reduced.

FIG. 11 is a block diagram of an electronic book. FIG. 11 includes abattery 1001, a power supply circuit 1002, a microprocessor 1003, aflash memory 1004, an audio circuit 1005, a keyboard 1006, a memorycircuit 1007, a touch panel 1008, a display 1009, and a displaycontroller 1010.

Here, the semiconductor device described in any of the above embodimentscan be used for the memory circuit 1007 in FIG. 11. The memory circuit1007 has a function of temporarily holding the contents of a book. Forexample, when a user uses a highlight function, the memory circuit 1007stores and holds data of a portion specified by the user. Note that thehighlight function is used to make a difference between a specificportion and the other portions while reading an electronic book, bymarking the specific portion, e.g., by changing the display color,underlining, making characters bold, changing the font of characters, orthe like. In order to store the data for a long time, the data may becopied to the flash memory 1004. Also in such a case, by employing thesemiconductor device described in any of the above embodiments, data canbe written and read at high speed and held for a long time, and powerconsumption can be sufficiently reduced.

As described above, the portable devices described in this embodimenteach incorporate the semiconductor device according to any of the aboveembodiments. Therefore, it is possible to obtain a portable device whichis capable of reading data at high speed, holding data for a long time,and reducing power consumption.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

This application is based on Japanese Patent Application serial no.2011-161329 filed with Japan Patent Office on Jul. 22, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising a memory cell, thememory cell comprising a transistor and a capacitor, wherein thetransistor comprises: an oxide semiconductor film; a source electrodelayer and a drain electrode layer over the oxide semiconductor film; agate insulating film over the oxide semiconductor film; and a gateelectrode layer over the gate insulating film, wherein the capacitorcomprises part of the oxide semiconductor film and one of the sourceelectrode layer and the drain electrode layer, wherein the part of theoxide semiconductor film and the one of the source electrode layer andthe drain electrode layer overlap with each other, wherein the gateelectrode layer of the transistor is electrically connected to a wordline, and wherein the other of the source electrode layer and the drainelectrode layer is electrically connected to a bit line.
 3. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor film comprises a channel formation region, a pair of firstregions with the channel formation region therebetween, and a pair ofsecond regions with the channel formation region therebetween, whereinone of the pair of first regions and one of the pair of second regionsare stacked, and wherein the other of the pair of first regions and theother of the pair of second regions are stacked.
 4. The semiconductordevice according to claim 2, wherein the oxide semiconductor filmcomprises a channel formation region, a pair of first regions with thechannel formation region therebetween, and a pair of second regions withthe channel formation region therebetween, wherein one of the pair offirst regions and one of the pair of second regions are stacked, whereinthe other of the pair of first regions and the other of the pair ofsecond regions are stacked, and wherein a resistance value of the pairof first regions is higher than a resistance value of the pair of secondregions.
 5. The semiconductor device according to claim 2, wherein theoxide semiconductor film comprises indium and zinc.
 6. The semiconductordevice according to claim 2, wherein the oxide semiconductor filmcomprises a carrier concentration being less than or equal to1×10¹²/cm³.
 7. The semiconductor device according to claim 2, whereinthe part of the oxide semiconductor film functions as a dielectric ofthe capacitor.
 8. The semiconductor device according to claim 2, whereinthe capacitor further comprises a wiring layer in contact with a bottomsurface of the part of the oxide semiconductor film.
 9. A semiconductordevice comprising a memory cell, the memory cell comprising a transistorand a capacitor, wherein the transistor comprises: an oxidesemiconductor film; a source electrode layer and a drain electrode layerover the oxide semiconductor film; a gate insulating film over the oxidesemiconductor film; and a gate electrode layer over the gate insulatingfilm, wherein the capacitor comprises: part of the oxide semiconductorfilm; an insulating film over the part of the oxide semiconductor film;and an electrode layer over the insulating film, and wherein the part ofthe oxide semiconductor film, the insulating film of the capacitor, andthe electrode layer of the capacitor overlap with each other.
 10. Thesemiconductor device according to claim 9, wherein the oxidesemiconductor film comprises a channel formation region, a pair of firstregions with the channel formation region therebetween, and a pair ofsecond regions with the channel formation region therebetween, whereinone of the pair of first regions and one of the pair of second regionsare stacked, and wherein the other of the pair of first regions and theother of the pair of second regions are stacked.
 11. The semiconductordevice according to claim 9, wherein the oxide semiconductor filmcomprises a channel formation region, a pair of first regions with thechannel formation region therebetween, and a pair of second regions withthe channel formation region therebetween, wherein one of the pair offirst regions and one of the pair of second regions are stacked, whereinthe other of the pair of first regions and the other of the pair ofsecond regions are stacked, and wherein a resistance value of the pairof first regions is higher than a resistance value of the pair of secondregions.
 12. The semiconductor device according to claim 9, wherein theoxide semiconductor film comprises indium and zinc.
 13. Thesemiconductor device according to claim 9, wherein the oxidesemiconductor film comprises a carrier concentration being less than orequal to 1×10¹²/cm³.
 14. The semiconductor device according to claim 9,wherein the part of the oxide semiconductor film functions as adielectric of the capacitor.
 15. The semiconductor device according toclaim 9, wherein the capacitor further comprises a second electrodelayer in contact with a bottom surface of the part of the oxidesemiconductor film.